As highly heat-resistant, highly rigid polymers, aromatic polyamides are known. The aromatic polyamides are polymers useful as engineering materials due to their high heat resistance and high mechanical strength. In particular, aromatic polyamides composed of para-directed aromatic groups represented by polyparaphenylene terephthalamide (hereinafter, referred to as PPTA) are of great utility value because they can result in shaped articles superior in strength and elastic modulus as well as the aforementioned properties due to their high stiffness. A para-directed aromatic polyamide typified by PPTA, however, exhibits low solubilities in solvents and dissolves only in extremely restricted solvents such as sulfuric acid. There, therefore, are significant limitations in process. A solution thereof causes no serious problems when fibers are produced therefrom. When it is processed into a shaped article of two or more dimensions such as film, however, it must be processed by a special shaping technique because the solution exhibits an optical anisotropy. Therefore, improvement is required in this respect.
On the other hand, as an approach to improve solubility, introduction of structural units having a bridge such as oxygen or a methylene group is known in U.S. Pat. No. 4,075,172 and Japanese Patent Application Publication No. 52-98795. Generally, however, the introduction of such structural units will affect superior mechanical characteristics, such as Young's modulus and strength, inherent to para-directed aromatic polyamides. As another approach, aromatic polyamides having an aromatic nucleus with a chlorine atom introduced thereto has been proposed in Japanese Patent Application Publication Nos. 52-84246 and 54-106564. The monomers of such aromatic polyamides, however, are expensive and those polymers are not conformable to a current trend that halogen-containing polymers are not preferred.
As a highly heat-resistant, highly rigid polymer other than aromatic polyamides, an aromatic carbohydrazide is disclosed in Japanese Patent No. 2853117, which discloses that a film which can exhibit an extremely high Young's modulus even in one direction is obtained by stretching. This film, however, has a high moisture absorption inherently due to its polymer structure.
Moreover, U.S. Pat. No. 3,642,711 discloses a method for obtaining a highly heat-resistant polymer through thermal dehydration-cyclization of an aromatic carbohydrazide. This method is, however, disadvantageous in industrial aspect because the cyclization reaction needs temperatures as high as 350° C. under reduced pressure. In addition, a thermal cyclization reaction is problematic in that if the reaction time is short, the cyclization reaction will proceed insufficiently, whereas if the reaction time is long, side reactions such as a decomposition reaction will take place.
As an electrolyte membrane, Nafion (registered trademark) of E.I. du Pont de Nemours and Company is used widely. This is problematic in that the cost is very high and the heat-resisting temperature is low because of the use of fluororesin in the polymer. Hydrocarbon-based electrolyte membranes are problematic in that they are of low mechanical strength though they are advantageous is cost. Thus, inexpensive electrolyte membranes having a high heat-resisting temperature and a high mechanical strength are demanded.